Human gestation is divided into two developmental stages: the embryonic period, which extends from conception to the end of the eighth week, followed by the fetal period, which lasts until parturition. During the embryonic period, the developing embryo undergoes numerous changes in order to recreate the human blueprint and to establish the maternal connection necessary for its survival. The genesis of virtually all essential structures occur prior to the eighth week of development. During the fetal period, these structures grow and become more elaborate (see, for review, Moore, 1977, in "The Developing Human," second edition, W. B. Saunders Company, Philadelphia).
Very early in the embryonic period, the fertilized egg, or zygote, undergoes a number of cell divisions to form a ball of about 15 small cells, called the morula. The morula enters the uterus and develops an inner cavity, thereby becoming a blastocyst consisting of (1) an inner cell mass which gives rise to the embryo; (2) a blastocyst cavity; and (3) an outer layer of cells, called the trophoblast. About five or six days after conception, the blastocyst attaches to the endometrial epithelium of the uterus and the trophoblastic cells invade the uterine wall.
With time, the actively erosive trophoblast invades the endometrial stroma, and the blastocyst is gradually engulfed by the endometrium (Id., p. 33). The trophoblast differentiates into two types: cytotrophoblast and syncytiotrophoblast. The syncytiotrophoblast is adjacent to the developing embryo and becomes a multinucleated protoplasmic mass in which no cell boundaries are discernible (Id., p. 34). Isolated spaces, called lacunae, appear in the syncytiotrophoblast at about day 9 and become filled with a fluid consisting of maternal blood and secretions. This fluid, or embryolymph, provides nutrition to the developing embryo and marks the beginning of the uteroplacental circulation. Eventually, the endometrium forms the maternal part, and the trophoblast forms the fetal part, of the placenta (Id., p. 36).
Once the primitive placental circulation is established, the embryo begins to develop at astonishing speed. By 20 days, the brain and spinal cord have begun to form. At about 22 days, the embryonic heart begins to beat. At 27 days arm and leg buds have appeared. By 30 days, the eyes and nose are forming. By 40 days, the arms are bent at the elbow, early fingers and ears are apparent, and the embryo is only one centimeter long.
This period of rapid development is accomplished by a carefully regulated program of cell division and differentiation which is, at this time, incompletely understood. An interest in the signals involved in initiating or terminating embryonic development has prompted analysis of hormones and cytokines associated with placental, embryonic, or fetal tissue. A brief list of some results of such studies follows.
Two well-documented protein products of the placenta are (1) human chorionic gonadotrophin (hCG) and (2) human chorionic somatomamotropin (hCS), also known as human placental lactogen (hPL) (Id., p. 105). hCG is produced by the trophoblast, and acts to prevent degeneration of the corpus luteum, an ovarian structure that produces progesterone. During early pregnancy, circulating hCG levels increase linearly until eight weeks of gestation, reaching a plateau at between nine and ten weeks of gestation,and thereafter declining until term. This secretory pattern is considered to be an important indicator of normal trophoblast development. In fact, if circulating hCG levels continue to increase beyond ten weeks of gestation, trophoblastic neoplasia, such as a hydatiform mole, should be suspected (Delf, 1957, Obstet. Gynecol. 9:1). On the other hand, a premature plateau and decrease in hCG level generally indicate early pregnancy failure (Aspillaga et al., 1982, Am. J. Obstet. Gynecol. 147:903).
Yoshida, Japanese Patent No. 59078694, May 7, 1984, reports the identification of a cobalt-activated substance in fetal or placental tissue which inhibits the action of a carcinogenic protein-forming enzyme.
Japanese Patent No. 2215730, Aug. 28, 1990, reports the isolation of the cytokine transforming growth factor-beta (TGF-beta) from human placenta.
Massague (1983, J. Biol. Chem. 258:13614-13620) reports the binding of epidermal growth-factor like transforming growth factor to epidermal growth factor receptors in human placenta membranes.
Roberts et al. (1985, Proc. Natl. Acad. Sci. U.S.A. 82:119-123) reports that TGF-beta may be isolated from human placenta, among other tissues. It was observed that the response of cells to TGF-beta appeared to be bifunctional, in that TGF-beta was able to stimulate reversible transformation of murine fibroblasts, but was also able to inhibit anchorage-dependent growth of normal rat kidney fibroblasts and of human tumor cells by increasing cell cycle time.
Letnansky (1987, Immunology 175:68) reports the isolation and characterization of a bovine placenta protein which specifically inhibits the proliferation of tumor cells. This protein, termed decidua inhibitory factor (DIF), was estimated to have a molecular weight of about 60 kD by SDS-PAGE.
Barnea et al. (1989, Placenta 10:331-344) report that human embryonal extracts modulate placental function in the first trimester. They observed that extracts of specific tissues were capable of decreasing or, alternatively, increasing hCG secretion. In particular, water extract of embryonal lung was found to produce a twofold decrease in hCG production by placental explants; a protein having a molecular weight less than 8000 daltons appeared to be the active agent, but was not purified.
Plowman et al. (1990, Mol. Cell. Biol. 10:1969-1981) report the isolation of a gene encoding amphiregulin (AR), a cytokine that is evolutionarily related to epidermal growth factor and to transforming growth factor alpha (TGF-alpha). AR was observed to be an 84 amino acid protein capable of acting as a bifunctional growth modulator, being able to promote the growth of normal epithelial cells and also to inhibit the growth of certain carcinoma cell lines. Human placenta and ovaries were found to express significant amounts of AR-encoding RNA. The unglycosylated AR precursor minus the signal peptide was predicted to have a molecular weight of about 25,942 D.
Further toward understanding the control of development, and in addition to the above mentioned hormones and cytokines, several lines of evidence suggest that there is an interdependence in mammals between the trophoblast and the embryo. Fetal death in both sheep and rats results in a decline in placental lactogen secretion (Ramsay et al., 1985, Biol. Neonate 47:42; Albrecht et al., 1984, Endocrinol. 107:766; Taylor et al., 1984, Res. Vet. Sci. 35:22; Robertson et al., 1984, Endocrinol. 114:22). Barnea et al. (supra) investigated the possible role of embryonic visceral organs as trophoblastic function regulators by examining the effects of dilute organ extracts upon in vitro secretion of hCG, and found an hCG-suppressive effect in certain fractions. It therefore appears that the developing embryo is not entirely subject to control by its environment; rather, the embryo itself appears to play an active role in the physiology of pregnancy.